Pictures: "Walking" Fish a Model of Evolution in Action

Share this gallery

Comment on this gallery

Pictures: "Walking" Fish a Model of Evolution in Action

1 / 5

Fish (Happily) Out of Water

Sometimes, a fish out of water doesn't feel like a fish out of water—at least if it's a Pacific leaping blenny (pictured). For the first time, scientists have closely studied the land-dwelling fish, which hops about the rocky coastlines of Micronesia (see map).

The new study revealed that the "walking" fish are amazingly agile on land, where they engage in complex social and courtship behaviors, study author Terry Ord, an evolutionary ecologist at the University of New South Wales in Australia, said by email.

But Ord and colleague S. Tonia Hsieh of Temple University also found that the landlubbers can forage, court, and mate—basically take care of all their blenny business—only during the few short hours of midtide. That's when the water level is high enough to keep the fish's skin wet but the waves aren't strong enough to carry the animals out to sea.

The blennies, which breathe through their gills and partly through their skin, will suffocate if they completely dry out. So "while these fish are very good at living on land ... they are nevertheless very constrained by their evolutionary history," noted Ord, whose study appeared in July in the journal Ethology. Hsieh has previously received funding for this research from the National Geographic Society.

A Pacific leaping blenny clings to a rock above the water, where the species is "completely comfortable," according to Ord.

For the study, Ord and his team observed and recorded the behaviors of individual fish then tried to catch the roughly 2.4-inch-long (6-centimeter-long) blennies to measure them and take photographs. The scientists "encouraged" the fish to jump into a small net by poking them with a pencil or hair pin.

"Invariably, they jump any place but into the net. Or if they do happen to jump into the net, they jump right back out again before you can put your hand over to stop them," Ord said.

"On a recent trip this year, one colleague of mine described trying to catch these fish as 'one of the most exasperating experiences in my life.' It does take practice, and it just goes to show you how apt to living on land these fish really are."

Pacific leaping blennies (pictured, a group hanging out on a rock) have elaborate social rituals, according to Ord. For instance, both males and females will aggressively flash their red dorsal fins at other fish-usually of the same sex-that are perceived as threats to their feeding sites.

"This display was often accompanied by a dramatic and rapid change in body color in both sexes, from dull green-gray to black," according to the study authors.

Courtship is equally as striking, Ord said. Males spend a vast amount of time nodding their heads near the entrances of small rock holes above the high-tide waterline, apparently enticing passing females to enter the holes.

"Those females that take a fancy to a male then inspect the male's rock hole and probably deposit eggs inside the hole, which the male then fertilizes and subsequently guards until the larvae hatch"—meaning male parental care is likely for this species.

Landlubbing fish (pictured, a Pacific leaping blenny out of water) aren't unique to Micronesia, Ord said. There are closely related species in Taiwan as well as reports of similar fish in the Philippines, the South Pacific, and the Atlantic.

This leads to an obvious question: "If these fish are terrestrial, does this mean these other fish have independently evolved a terrestrial lifestyle on these other islands, or did an evolutionary ancestor that was already terrestrial somehow disperse to these other islands?" Ord said.

"We simply don't know the answer yet, but we're in the process of combining genetic studies and behavior studies—like this current study—to figure it out."

Studying the Pacific leaping blenny (pictured) is key for two reasons, Ord said.

For one, little is known about how ancient fish first made the transition to land during the Devonian period, which began about 416 million years ago. The Pacific leaping blenny offers a rare opportunity to research a modern fish that has made a similar transition.

In addition, the fish is an ideal model for studying natural selection, or the adaptations that occur when species invade new environments or when their existing environments change.

"These fish have moved out of one environment for which they were well adapted—a water environment-to another environment that is the most extreme opposite you could possibly imagine, a land environment," Ord said.

"By studying this little fish that lives on land, we can study evolution and natural selection in action."